PROJECT SUMMERY Out-of-hospital cardiac arrest (OHCA) is a common, life-threatening event that is associated with poor patient outcomes. Only ~30% of patients survive to hospital admission and less than 10% are ultimately discharged alive. Yet as a recent Institute of Medicine (IOM) report highlighted, some communities have been consistently more successful in responding to and treating OHCA than others. While this is largely believed to be due to the adoption of key care processes by emergency medical services (EMS) agencies in the pre-hospital setting, how these processes are actually utilized in routine practice and the underlying factors contributing to their successful implementation is largely unknown. Further, an integrative systems approach that examines the interaction of EMS agencies with other important stakeholders who provide care before and after EMS arrives ? such as first-responders (e.g., police, fire) and hospitals ? has not been undertaken. A better understanding and dissemination of strategies to improve care processes employed by high-performing EMS agencies across the continuum of OHCA care is a critical yet unfulfilled step for improving outcomes. Accordingly, the Enhanced Pre-Hospital Outcomes for Cardiac Arrest (EPOC) study will lay the foundation for future quality improvement efforts in OHCA through the following three aims: (1) Identify top- and bottom-performing EMS agencies in survival for OHCA.; (2) Define `best practices' at top-performing EMS agencies; (3) Validate factors associated with high survival at top-performing EMS agencies. We will use a sequential mixed methods approach to accomplish these aims. We build on extensive prior work by our investigative team to systematically understand factors associated with delivering optimal care in other emergency settings (mass casualty incidents, heart attacks, in- hospital cardiac arrest) and leverage our team's multidisciplinary expertise in emergency care, cardiology, clinical epidemiology, and mixed methods research. Findings from this study will be used to construct a toolkit of `best practices' ? the EPOC Toolkit ? that may be implemented by EMS agencies both within and beyond CARES through key state and national partners to improve OHCA outcomes in communities across the U.S.